1. Field of the Invention
The present invention relates to a circuit for use in an FSK (Frequency Shift Keying) receiving apparatus such as a pager or the like for receiving a modulated signal (hereinafter referred to as FSK signal) obtained by FSK, and particularly relates to a base band signal offset correcting circuit for automatically correcting a DC offset of a demodulated base band signal, a method therefor, and an FSK receiving apparatus with such a correcting circuit.
2. Description of the Related Art
An FSK system is a method of transmitting a digital signal through frequency transition of a carrier. To demodulate an FSK signal in a receiving apparatus, there is adopted a method in which an output voltage of a frequency discriminating circuit obtained on the basis of the received FSK signal is compared with a reference voltage to perform signal judgement to thereby obtain a demodulated digital signal. In the FSK system, when there is generated a carrier frequency error on the transmission side, a frequency error of a frequency converting local oscillator on the receiving side, an error of a center frequency of the frequency discriminating circuit, or the like, such an error appears as a DC offset of the demodulated base band signal. This DC offset component becomes equivalent to a deviation in the reference voltage when signal judgement is performed on the demodulated base band signal. Thus, the DC offset component results in deterioration in the demodulation characteristic in the receiving apparatus. Particularly, when a multi-valued modulated signal such as a four-values FSK signal or the like is received, it is necessary to provide a plurality of reference voltages close to each other. It is therefore necessary to detect the above-mentioned DC offset component with high precision and correct it.
To correct such an error as described above, there are a method of correcting an input frequency error in the frequency discriminating circuit and a method of directly correcting the DC offset with the demodulated base band signal itself If there is room in a bandwidth of an intermediate frequency band, the method of correcting the DC offset only with the demodulated base band signal is more advantageous in that the circuit configuration can be made simply. Thus, there have been considered various methods for detecting a DC offset of a demodulated base band signal in a base band so as to correct a reference voltage. Examples of the methods include: (a) a method in which upper and lower limit values of a demodulated base band signal are detected and a DC component is detected from an average value of those limit values; (b) a method in which a modulated base band signal and an output signal of a signal judging comparator are trued up in their amplitudes and a difference between those signals is integrated so that a DC component is detected; (c) a method in which a demodulated base band signal is integrated so that a DC component is detected; and so on.
Of the above-mentioned correcting methods, in the method (a), an error may occur easily due to noise in the detection of the upper and lower limit values. On the other hand, the methods (b) and (c) have an advantage that the influence of noise can be reduced by integration. Particularly, in the method (c), the circuit configuration is simplest because it will go well if only an integrating circuit is provided as a correcting circuit. When a correcting circuit is configured by use of the method (c), an output voltage of the integrating circuit is used as a reference voltage of a comparator so as to correct a DC offset. It is, however, necessary to increase a time constant of the integrating circuit in order to detect the DC offset component with high precision, so that the convergence time of the output voltage is elongated. As a result, it takes a long preheating time (readiness period of time) for the circuit to run. In addition, there is a problem that receiving operation cannot be started because divergence appears in the reference voltage till the integration has been finished and the output voltage has converged.
As mentioned above, in the base band signal offset correcting circuit in the background-art FSK receiving apparatus, there is a problem that it is impossible to deal with high-speed operation because it is necessary to increase the time constant of the integrating circuit in order to detect a DC offset component with high precision. A time division multiple access (TDMA) system is adopted widely in digital mobile communication. Since burst reception is performed for receiving a signal intermittently in communication based on this TDMA system, a high-speed property is required in the correcting circuit.
The present invention has been developed in consideration of the foregoing situation. It is an object of the present invention to provide a base band signal offset correcting circuit and a method therefor, in which it is possible to detect a DC offset component of a demodulated base band signal easily with high precision and at a high speed, and it is possible to perform correction at a high speed and with high precision. It is another object to provide an FSK receiving apparatus with such a correcting circuit.
According to one aspect of the present invention, there is provided a base band signal offset correcting circuit for correcting a DC offset of a demodulated base band signal which is obtained by demodulating an FSK signal, comprising: an integrating circuit for integrating said demodulated base band signal; offset detecting means for judging whether an output of said integrating circuit is or not within a predetermined range to thereby detect a DC offset component of said demodulated base band signal; counting means for incrementing or decrementing an output of said offset detecting means at predetermined timing to thereby update a count value of said counting means; reference voltage generating means for generating a reference voltage on the basis of said count value, said reference voltage being used in a comparator to perform signal judgement upon said demodulated base band signal; and initial integral value setting means for supplying said reference voltage generated the last time to said integrating circuit to thereby set said reference voltage as an initial output value of said integrating circuit.
According to further aspect of this invention, the base band signal offset correcting circuit, further comprises: frequency error judging means for comparing said count value of said counting means with a reference value to thereby detect a frequency error of said FSK signal; second counting means for incrementing or decrementing an output of said frequency error judging means at predetermined timing to thereby update a count value of said second counting means; and local oscillation frequency control means for controlling an oscillation frequency of a local oscillator in accordance with said count value of said second counting means.
According to another aspect of this invention, there is provided an FSK receiving apparatus with a base band signal offset correcting circuit for correcting a DC offset of a demodulated base band signal which is obtained by demodulating an FSK signal, comprising: integrating circuit for integrating the demodulated base band signal; offset detecting means for judging whether an output of the integrating circuit is or not within a predetermined range to thereby detect a DC offset component of the demodulated base band signal; counting means for incrementing or decrementing an output of the offset detecting means at predetermined timing to thereby update a count value of the counting means;
reference voltage generating means for generating a reference voltage on the basis of the count value, the reference voltage being used in a comparator to perform signal judgement upon the demodulated base band signal; and initial integral value setting means for supplying the reference voltage generated the last time to the integrating circuit to thereby set the reference voltage as an initial output value of the integrating circuit.
According to yet another aspect of this invention, in the FSK receiving apparatus, the base band signal offset correcting circuit comprising: frequency error judging means for comparing the count value of the counting means with a reference value to thereby detect a frequency error of the FSK signal; second counting means for incrementing or decrementing an output of the frequency error judging means at predetermined timing to thereby update a count value of the second counting means; and local oscillation frequency control means for controlling an oscillation frequency of a local oscillator in accordance with the count value of the second counting means.
In still further aspect of the present invention, there is provided a base band signal offset correcting method for correcting a DC offset of a demodulated base band signal which is obtained by demodulating an FSK signal, comprising: an integrating step for integrating the demodulated base band signal; an offset detecting step for judging whether an integral value of the demodulated base band signal is or not within a predetermined range to thereby detect a DC offset component of the demodulated base band signal; a counting step for incrementing or decrementing the DC offset component at predetermined timing to thereby update a count value; a reference voltage generating step for generating a reference voltage on the basis of the count value, the reference voltage being used in a comparator to perform signal judgement upon the demodulated base band signal; and an initial integral value setting step for supplying the reference voltage generated the last time to an integrating circuit for carrying out the integrating step to thereby set the reference voltage as an initial output value of the integrating circuit.
In still further aspect of this invention, the base band signal offset correcting method comprises: a frequency error judging step for comparing the count value in the counting step with a reference value to thereby detect a frequency error of the FSK signal; a second counting step for incrementing or decrementing the detected frequency error at predetermined timing to thereby update a count value; and a local oscillation frequency control step for controlling an oscillation frequency of a local oscillator in accordance with the count value in the second counting step.
In the base band signal offset correcting circuit, in the method therefor and in the FSK receiving apparatus with the correcting circuit according to the present invention, a demodulated base band signal is integrated by an integrating circuit, and it is judged whether an integrated value of the demodulated base band signal is or not within a predetermined range, so that a DC offset component of the demodulated base band signal is detected.
Then, the output of the offset detecting means is incremented or decremented at predetermined timing in the counting means so that a count value is updated. A reference voltage for a comparator for performing signal judgement upon the demodulated base band signal is generated by reference voltage generating means, on the basis of the count value. Then, the reference voltage generated the last time is supplied to the integrating circuit through initial integral value setting means so that, for example, a capacitor of the integrating circuit is charged to set initial output value of the integrating circuit. Thus, the convergence time of the integrating circuit is shortened so that the DC offset of the demodulated base band signal can be corrected at a high speed and with high accuracy.
In addition, the count value of the aforementioned counting means is compared with a reference value in frequency error judging means so that a frequency error of the FSK signal is detected. Then, the output of the frequency error judging means is incremented or decremented in second counting means at predetermined timing so that a count value is updated. In accordance with this count value, an oscillation frequency of a local oscillator is controlled by local oscillation frequency control means. Thus, while the DC offset of the demodulated base band signal is corrected, the frequency control of the local oscillator can be performed on the basis of the result of the correction. It is therefore possible to correct the DC offset of the demodulated base band signal with high precision even if the frequency control is rough in resolution.